1. Field of the Invention
The present invention generally concerns magnetic resonance tomography (MRT) as employed in medicine for examination of patients. The present invention in particular concerns a method as well as an MRT system for implementation of such a method making use of PPA based, partial parallel acquisition.
2. Description of the Prior Art
MRT is based on the physical phenomenon of nuclear magnetic resonance and has been successfully used as an imaging method for over 20 years in medicine and biophysics. In this examination modality, the subject is exposed to a strong, constant magnetic field. The nuclear spins of the atoms in the subject, which were previously randomly oriented, thereby align. Radio-frequency energy can now excite these “ordered” nuclear spins to a specific oscillation. In MRT, this oscillation generates the actual measurement signal, which is acquired by suitable reception coils. By the use of inhomogeneous magnetic fields generated by gradient coils, the measurement subject can be spatially coded in all three spatial directions. The method allows a free selection of the slice to be imaged, so slice images of the human body can be acquired in all directions. MRT as a tomographic imaging method in medical diagnostics is distinguished predominantly as a “non-invasive” examination method with a versatile contrast possibility. Due to the excellent ability to represent soft tissue, MRT has developed into a method superior in many ways to x-ray computed tomography (CT). MRT today is based on the application of spin echo and gradient echo sequences that enable an excellent image quality with measurement times in the range of seconds to minutes.
The acquisition of the data in MRT occurs in k-space (frequency domain). The MRT image in the image domain is linked with the MRT data in k-space by means of Fourier transformation. The spatial coding of the subject, which spans k-space, can occur in various manners; but a Cartesian or a per-projection sampling is the most common. The coding ensues by means of gradients in all three spatial directions.
The radio-frequency excitation of the subject can be made selective to the volume on the basis of similar spatial coding by the use of gradient fields during the excitation. The spatially-varying strength of the excitation, i.e. the flip angle dependent on the location, corresponds for small flip angles as a first approximation analogous to the reception case of the Fourier-transformed RF signal in transmission k-space. A temporally-efficient, volume-selective excitation was previously only possible in a spatial direction, i.e. in the form of slice selection, since the corresponding k-space trajectory corresponds to a single line in 3D k-space.
Multi-dimensional, volume-selective excitations require the spanning of multi-dimensional k-space trajectories. In a manner analogous to 2D and 3D phase coding in the reception case, this requires a great deal of time and prevents the application of volume-selective excitation, for example in spectroscopy or for homogenization of the flip angle distribution in intense field apparatuses.
The most effective methods for shortening the image measurement time for the reception of MR signals given Cartesian sampling are based on a reduction of the number of time-consuming phase coding steps Ny and the use of a number of signal acquisition coils, known as a “partial parallel acquisition” and designated herein as PPA. This principle can be transferred to data acquisition methods with radial or spiral-shaped sampling, by reducing the number of time-consuming angle steps Nφ or the number or the length of the spiral arms. In the following, a Cartesian k-space sampling is considered without limitation of the generality in the transmission and reception case. In order to differentiate between transmission and reception k-space trajectories, the former is designated with the symbol κ (kappa).
The fundamental idea in conventional, reception-side PPA imaging is that the k-space data are not acquired by a single coil, but rather by component coils arranged linearly, annularly or matrix-like around the subject, for example in the form of a coil array. As a result of their geometry, each of the spatially-independent coils of the array supplies certain spatial information which is used in order to achieve a complete spatial coding by a combination of the simultaneously-acquired coil data. This means that a number of “omitted” lines in k-space can also be determined from a single acquired k-space line.
Receiver-side PPA methods thus use spatial information that is contained in the signals from the components of a coil arrangement in order to partially replace the time-consuming relaying of the phase coding gradient. The image measurement time is thereby reduced corresponding to the ratio of number of the lines of the reduced data set to the number of the lines of the conventional (thus complete) data set. In comparison to the conventional acquisition, in a typical PPA acquisition only a fraction (½, ⅓, ¼, etc.) of the k-space lines are acquired. A special reconstruction is then applied to the k-space data in order to reconstruct the missing lines, and thus to obtain the full field of view (FOV) image in a fraction of the time. The FOV is established according to the factor 2π/k by the size of k-space under consideration.
Established PPA methods for Cartesian data acquisition such as SENSE or GRAPPA make use of the Fourier shift theorem, whereby an additional phase Δky y is impressed on the nuclear magnetic resonance signal along the phase coding direction by a combination of the individual coil signals. New ky lines that no longer have to be explicitly measured thus arise in the frequency domain, so the measurement time is reduced.
In all PPA methods, additional calibration data points are necessarily also acquired (additionally measured central reference lines) that are added to the actual measurement data, and a reduced data set can actually be completed again only on the basis of such calibration data points.
Recently, transmitter-side PPA imaging methods have also been suggested that additionally enable an accelerated volume-selective excitation. A number of simultaneously-operated transmission coils are required for this that, are arranged around the subject to be examined to form a PPA transmission coil array. Transmission coil-side acceleration is achieved (analogous to the accelerated receiver-side PPA data acquisition along an under-sampled reception trajectory in reception k-space) by the excitation of the region to be examined in transmission k-space ensuing along under-sampled transmission trajectories, but which generate an excitation profile that corresponds to that of the complete transmission trajectory. For this purpose, individually-determined RF pulse shapes must be used in the individual elements of the transmission coil array. One possibility for this determination is described in “Transmit SENSE” (U. Katscher, P. Bornert, C. Leussler, J S. van den Brink, “Transmit SENSE”, Magnetic Resonance in Medicine, 2003 January; 49(1): 144-150) and exhibits parallels to the receiver-side SENSE method. It has the disadvantage that the sensitivity profiles of all participating transmission coils (transmission coil sensitivities) must be known. The measurement-technical determination of these transmission sensitivities represents a central problem since these cannot be measured independent of the reception sensitivities. Even given the use of the same radio-frequency coils for excitation and for reception, it cannot be assumed that the transmission coil sensitivities and the receiver coil sensitivities are identical, since the transmission field and the reception field differ significantly, in particular at high field strengths.